contrib: testgen: remove redundant base58 implementation

Summary:
The redundant base58.py was old code with horrible hacks to coerce python 3 builtin functions to behave the python 2 way.

This is a backport of core#24576
Depends on D18904

Test Plan:

contrib/testgen/gen_base58_test_vectors.py invalid 50
contrib/testgen/gen_base58_test_vectors.py valid 50

ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18914